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    Biasing - Original Dynaco ST-70 and the VTA ST-70

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    Bob Latino
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    Location : Massachusetts

    Biasing - Original Dynaco ST-70 and the VTA ST-70

    Post by Bob Latino on Thu Jun 19, 2014 8:08 pm

    I had someone ask me by Email "why the original Dynaco ST-70 uses 10K bias pots and why does the VTA ST-70 use 50K bias pots?"  I thought that the answer might be good for the forum.

    1. In the original Dynaco ST-70 you had ONE pot used to bias TWO output tubes at once while on the VTA ST-70 you have TWO pots to bias TWO output tubes (on each channel). The use of the lower value 10K pots allows you to crank in more negative DC voltage to handle the TWO output tubes. The down side of using the 10K pot on the original Dynaco ST-70 is that the amount of RANGE that you have is much less. A common problem on original Dynaco ST-70's is that when you replace the selenium rectifier with a diode, the diode has more forward voltage and what happens is that that 10K pot now does not have ENOUGH RANGE to handle EL34 output tubes. This problem is easily solved though by changing out the two 10K resistors on the 7 lug terminal strip to smaller values. The use of two 5K resistors in place of the 10K resistors is a common solution to the problem. You can also parallel two more 10K resistors over the 10K's that are already there as a solution (giving you two equivalent 5K resistors).

    2. The use of the 50K pots on the VTA ST-70, 120 and M-125's gives you EXTRA RANGE. On the VTA ST-70 you can use EL34, 6L6, 5881, KT66, KT77, KT88 or 6550 output tubes. On the VTA ST-120 you can use KT88, 6550 or KT120 output tubes. On the M-125's you can use the same three output tubes in EITHER two output tube mode OR four output tube mode and still have enough range to bias the tubes properly. The only downside to the 50K bias pots is that because of the great range the bias pots are a little "touchy" and it is near impossible to set the bias on an EL34 output tube to .400 volts DC and get the last digit to actually read zero - BUT - you don't really need to get the last digit a zero. If all the EL34 output tubes on your VTA ST-70 are between .390 and .410 VDC, you are fine. Understand also that if your line voltage goes up or down even a volt or two, the bias will rise with a rising line voltage and drop as your line voltage goes down.

    Bob

    tubes4hifi
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    Posts : 1292
    Join date : 2008-11-30

    Re: Biasing - Original Dynaco ST-70 and the VTA ST-70

    Post by tubes4hifi on Thu Jun 19, 2014 8:21 pm

    also . . . all four of those 50K pots are in parallel with each other, so the equivalent of 12.5K overall . . .

    scottlowe

    Posts : 38
    Join date : 2014-04-05

    Re: Biasing - Original Dynaco ST-70 and the VTA ST-70

    Post by scottlowe on Fri Jun 20, 2014 1:02 pm

    I know this is off subject .but can i run just one tube per channel on the st-120? and if it's ok to do so would they be in the two sockets near to or farthest from the transformer .and do they still bias @ .550 .Thank you and hope its not to far off subject.Scott

    tubes4hifi
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    Join date : 2008-11-30

    Re: Biasing - Original Dynaco ST-70 and the VTA ST-70

    Post by tubes4hifi on Fri Jun 20, 2014 1:17 pm

    it's a push-pull amplifier, so running one tube per side you'll get push or pull, only half the music!! NOPE - won't work!!!

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